The Prophet ﷺ used to stand in prayer at night until his feet became cracked.Ayesha asked, "Why do you do this, Messenger of Allah, when Allah has forgiven you your past and future sins?"He replied, "Should I not love to be a grateful servant?"When he grew heavier in body, he would pray sitting down, and when he wanted to bow, he would stand up, recite, and then bow.
أن النبي ﷺ كان يقوم من الليل حتى تتفطر قدماه، فقالت عائشة: لم تصنع هذا يا رسول الله، وقد غفر الله لك ما تقدم من ذنبك وما تأخر؟ قال: أفلا أحب أن أكون عبدا شكورا، فلما كثر لحمه صلى جالسا، فإذا أراد أن يركع قام فقرأ، ثم ركع.
Bukhari narrated it in the tafsir of Surah Fath (4837), and Muslim in the Book of the Description of the Day of Resurrection (2820), both from the hadith of Urwa bin Zubayr, from Ayesha, and the wording given is Bukhari's. Her saying, "until his feet would crack," means they would split open.